1. Field of the Invention
The present invention generally relates to a magnetoresistive device and a magnetoresistive head. More particularly, the present invention relates to a magnetoresistive device in which a magnetoresistance is greatly changed in a low magnetic fields, and to a magnetoresistive head which is configured using such a magnetoresistive device and is suitable for high-density magnetic recording and reproducing operations.
2. Description of the Related Art
A magnetoresistive sensor (hereinafter, simply referred to as an xe2x80x9cMR sensorxe2x80x9d) and a magnetoresistive head (hereinafter, simply referred to as an xe2x80x9cMR headxe2x80x9d) using a magnetoresistive device have been under development. Conventionally, a permalloy made of Ni0.8Fe0.2 and an alloy film made of Ni0.8Co0.2 are mainly used as magnetic materials for these devices. The ratio of change in magnetoresistance (hereinafter, simply referred to as an xe2x80x9cMR ratioxe2x80x9d) of these magnetoresistive materials is about 2.5%. In order to develop a magnetoresistive device exhibiting a higher sensitivity, a magnetoresistive material having a higher MR ratio is required.
It was recently found that [Fe/Cr] and [Co/Ru] multilayers in which an antiferromagnetic coupling is attained via a metal non-magnetic thin film made of Cr or Ru exhibit a giant magnetoresistance effect in a ferromagnetic field (about 1 to about 10 kilo-oersteds (kOes)) (Physical Review Letter Vol. 61, p. 2472, 1988; and Physical Review Letter Vol. 64, p. 2304, 1990).
However, since these artificial multilayers require a magnetic field having an intensity of several to several tens of kOes in order to obtain a large MR change, such artificial multilayers cannot be practically applied to a magnetic head and the like.
In addition, it was also found that an [Nixe2x80x94Fe/Cu/Co] artificial multilayer using magnetic thin films made of Nixe2x80x94Fe and Co having different coercivities in which they are separated by a metal non-magnetic thin film made of Cu and which are not magnetically coupled to each other exhibits a giant magnetoresistance effect, and a magnetoresistive material which has an MR ratio of about 8% when a magnetic field an intensity of about 0.5 kOe is applied at room temperature was obtained (Journal of Physical Society of Japan, Vol. 59, p. 3061, 1990).
However, in the case of using a magnetoresistive material of such a type, a magnetic field having an intensity of about 100 Oes is required for obtaining a large MR change. Moreover, the magnetoresistance thereof asymmetrically varies from the negative magnetic field to the positive magnetic field. i.e. the magnetoresistance thereof exhibits a poor linearity. Thus, such a magnetoresistive material has characteristics which are not suitable for practical use.
Moreover, it was also found that [Nixe2x80x94Fexe2x80x94Co/Cu/Co] and [Nixe2x80x94Fexe2x80x94Co/Cu] artificial multilayers using magnetic thin films made of Nixe2x80x94Fexe2x80x94Co and Co in which an RKKY-type antiferromagnetic coupling is attained via Cu exhibit a giant magnetoresistance effect, and a magnetoresistive material which has an MR ratio of about 15% when a magnetic field having an intensity of about 0.5 kOe is applied at room temperature was obtained (Technical Report by THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS of Japan, MR91-9).
However, in the case of using a magnetoresistive material of such a type, the magnetoresistance thereof varies substantially linearly from the zero magnetic field to the positive magnetic field and the material has properties which are sufficiently suitable for the application to an MR sensor. Nevertheless, in order to obtain a large MR change, a magnetic field having an intensity of about 50 Oes is also required. Thus, such a property is not appropriate for the application to an MR head which is required to be operated at most at about 20 Oes and preferably less.
As a film which can he operated even when a very weak magnetic field is applied, a spin-valve type film in which Fexe2x80x94Mn as an antiferromagnetic material is attached to a structure of Nixe2x80x94Fe/Cu/Nixe2x80x94Fe has been proposed (Journal of Magnetism and Magnetic Materials 93, p. 101, 1991). The operating magnetic field of a magnetoresistive material of this type is actually weak, and a good linearity is observed. However, the MR ratio thereof is as small as about 2%, and the Fexe2x80x94Mn film has poor corrosion resistance and a low Neel temperature (ordering temperature). Consequently, the properties of such a device disadvantageously exhibit a great temperature dependence.
Furthermore, a spin-valve film having a structure of Nixe2x80x94Fe/Cu/Coxe2x80x94Pt or the like using a hard magnetic material such as Coxe2x80x94Pt instead of an antiferromagnetic material has also been proposed. In such a case, a parallel magnetization state and an anti-parallel magnetization state are created by rotating the magnetization direction of a soft magnetic layer at a coercivity equal to or less than that of a hard magnetic layer. However, even when such a structure is employed, it is still difficult to improve the properties of the soft magnetic layer. Thus, this structure has not been used practically, either.
Moreover, a structure such as Cu/Nixe2x80x94Fe/Cu/Nixe2x80x94Fe/Fexe2x80x94Mn formed by attaching a low-resistance back layer, made of a metal having a low resistance, to the back of a spin-valve film has also been proposed as a means for increasing the MR ratio of a spin valve film (U.S. Pat. No. 5,422,571). Such a structure is an attempt to increase the MR ratio by lengthening the mean free path of the electrons having a particular spin direction.
A conventional spin-valve type MR device, no matter whether the device is of the type using an antiferromagnetic material or of the type using a hard magnetic layer, had a problem in that the MR ratio thereof is low, even though the magnetic field sensitivity thereof is excellent. Similarly, the MR ratio cannot be satisfactorily increased even when the low-resistance back layer is provided. This is presumably because a small thickness of a spin-valve type MR device is likely to cause the diffusive scattering of electrons on the surface of the device.
Such a phenomenon can be explained in more detail as follows.
A giant magnetoresistance effect originally results from the spin-dependent scattering of electrons at an interface between a magnetic layer and a non-magnetic layer. Thus, in order to increase the possibility of the scattering generation, it is important to lower the possibility of the non-spin-dependent scattering generation and to lengthen the mean free path of electrons. In a spin-valve film, the number of magnetic layers and non-magnetic layers to be stacked is small. Thus, the film thickness of a spin-valve film is generally smaller (e.g., in the range from about 20 nm to about 50 nm) than that of an antiferromagnetic coupling type giant magnetoresistive film. Consequently, the possibility that electrons are scattered on the surface of such a film is high, and the mean free path of electrons is short. This is the principal reason why the MR ratio of a spin-valve film becomes low.
Ordinarily, the surface of a thin film has some unevenness on the order of several tenths of a nm which is substantially on the same order of the wavelength of conduction electrons (i.e., a Fermi wavelength). In such a case, the conduction electrons are subjected to diffusive scattering on the surface of the film. In general, in the case of a diffusive scattering, the spin direction of electrons is not maintained.
The magnetoresistive device of the present invention includes: at least two magnetic layers stacked via a non-magnetic layer therebetween; and a metal reflective layer of conduction electrons formed so as to be in contact with at least one of outermost two layers of the magnetic layers. the metal reflective layer being in contact with one surface of the outermost magnetic layer which is opposite to the other surface of the outermost magnetic layer in contact with the non-magnetic layer, the metal reflective layer being likely to reflect conduction electrons while maintaining a spin direction of electrons.
In one embodiment, the magnetoresistive device further includes a non-magnetic layer between the metal reflective layer and the magnetic layer.
In another embodiment, the non-magnetic layers are mainly composed of Cu, and the metal reflective layer is mainly composed of at least one of Ag, Au, Bi, Sn and Pb.
In still another embodiment, the magnetic layer in contact with the metal reflective layer via the non-magnetic layer is mainly composed of a Co-rich Coxe2x80x94Fe alloy.
In still another embodiment, the magnetic layer includes at least two layers of a magnetic layer and an or a Co-rich Coxe2x80x94Fe alloy, the interface magnetic layer being in contact with the metal reflective layer via the non-magnetic layer.
In still another embodiment, the magnetic layer in contact with the metal reflective layer via the non magnetic layer includes at least two interface magnetic layers which sandwich a soft magnetic layer therebetween and are mainly composed of Co or a Co-rich Coxe2x80x94Fe alloy.
In still another embodiment, the metal reflective layer has a smooth surface.
In still another embodiment, at least a part of the surface of the metal reflective layer is smooth on the order of tenths of a nm.
In still another embodiment, at least 10% of the surface of the metal reflective layer is a smooth surface having an unevenness of about three angstrom or less.
In still another embodiment, the magnetic layer directly in contact with the metal reflective layer is mainly composed of Co or a Co-rich Coxe2x80x94Fe alloy.
In still another embodiment, the magnetic layer includes at least two layers of a magnetic layer and an interface magnetic layer which is mainly composed of Co or a Co-rich Coxe2x80x94Fe alloy, the interface magnetic layer being directly in contact with the metal reflective layer.
In still another embodiment, the magnetic layer directly in contact with the metal reflective layer includes at least two interface magnetic layers which sandwich a soft magnetic layer therebetween and are mainly composed of Co or a Co-rich Coxe2x80x94Fe alloy.
In still another embodiment, at least one of the at least two magnetic layers has a different coercivity from a coercivity of the other magnetic layer(s).
In still another embodiment. the magnetoresistive device includes: a first and a second magnetic layer which are stacked via a non-magnetic layer; an antiferromagnetic layer formed in contact with a surface of the first magnetic layer which is opposite to the other surface of the first magnetic layer in contact with the non-magnetic layer; and a metal reflective layer formed in contact with a surface of the second magnetic layer which is opposite to the other surface of the second magnetic layer in contact with the non-magnetic layer.
In still another embodiment, the antiferromagnetic layer is an oxide.
In still another embodiment, the antiferromagnetic layer is made of Nixe2x80x94O.
In still another embodiment, the antiferromagnetic layer is made of xcex1-Fe2O3.
In still another embodiment, the second magnetic layer includes two or more magnetic layers which are stacked via a non-magnetic layer.
In still another embodiment, the antiferromagnetic layer is epitaxially grown over a substrate.
In still another embodiment, the magnetoresistive device includes a structure in which a first magnetic layer, the non magnetic layer, a second magnetic layer, an antiferromagnetic layer and the metal reflective layer are stacked in this order.
In still another embodiment, the magnetoresistive device further includes a non-magnetic layer between the antiferromagnetic layer and the metal reflective layer.
In still another embodiment, the antiferromagnetic layer is made of an Irxe2x80x94Mn alloy.
In still another embodiment, the magnetoresistive device includes a structure in which a first antiferromagnetic layer, a magnetic layer, a non-magnetic layer, a soft magnetic layer, a non-magnetic layer, a magnetic layer, a second antiferromagnetic layer and a metal reflective layer are stacked in this order directly on a substrate or over the substrate via an underlying layer.
In still another embodiment, the magnetoresistive device further includes a non-magnetic layer between the second antiferromagnetic layer and the metal reflective layer.
In still another embodiment, the second antiferromagnetic layer is made of an Irxe2x80x94Mn alloy.
In still another embodiment, the first antiferromagnetic layer is an oxide.
In still another embodiment, the first antiferromagnetic layer is made of Nixe2x80x94O.
In still another embodiment, the soft magnetic layer includes two or more magnetic layers which are stacked via a non-magnetic layer.
In still another embodiment, at least one of the first and the second antiferromagnetic layers are made of an Irxe2x80x94Mn alloy.
In still another embodiment, the first antiferromagnetic layer is made of xcex1-Fe2O3.
In still another embodiment, the first antiferromagnetic layer is epitaxially grown over the substrate.
In still another embodiment, the magnetoresistive device includes a structure in which a metal reflective layer, a first antiferromagnetic layer, a magnetic layer, a non-magnetic layer, a soft magnetic layer, a non-magnetic layer, a magnetic layer, a second antiferromagnetic layer and a metal reflective layer are stacked in this order directly on a substrate or over the substrate via an underlying layer.
In still another embodiment, the magnetoresistive device further includes a non-magnetic layer between the first antiferromagnetic layer and the metal reflective layer and/or between the second antiferromagnetic layer and the metal reflective layer.
In still another embodiment, the non-magnetic layer is epitaxially grown over the substrate.
In still another embodiment, a (100) plane of the non-magnetic layer is epitaxially grown vertically to a growth direction of thin layers.
In still another embodiment, the non-magnetic layer is epitaxially grown over an Mgo (100) substrate via a Pt underlying layer.
The magnetoresistive head of the present invention includes: a magnetoresistive device including at least two magnetic layers which are stacked via a non magnetic layer therebetween, and a metal reflective layer of conduction electrons formed so as to be in contact with at least one of outermost two layers of the magnetic layers, the metal reflective layer being in contact with a surface of the outermost magnetic layer which is opposite to the other surface of the outermost magnetic layer in contact with the non-magnetic layer, the metal reflective layer being likely to reflect conduction electrons while maintaining a spin direction of electrons; and a lead portion for supplying current to the magnetoresistive device. A magnetization easy axis of a magnetic layer having a smallest coercivity of the magneto-resistive device or a magnetization easy axis of a magnetic layer not in contact with an antiferromagnetic layer is vertical to a direction of a signal magnetic field to be detected.
Hereinafter, the functions or the effects to be attained by the present invention will be described.
The magnetoresistive device of the present invention is characterized by including a metal reflective layer, which is likely to cause a specular scattering while maintaining the spin direction of electrons, on the surface of a spin-valve film.
The metal reflective layer is required to have a surface which can be regarded as smooth on the order of several tenths of a nm. In such a case, conduction electrons generate an elastic scattering (specular scattering) at the surface of a film, the spin direction of the conduction electrons is reserved, and the same effects as those attained when the mean free path thereof has become longer can be attained. As a result, the MR ratio is increased.
The metal reflective layer is preferably made of a material such as Ag, Au, Bi, Sn or Pb. These materials are likely to contribute to a smooth surface on the order of several tenths of a nm, unlike the materials such as Ni, Fe, Cu and Co which are frequently used for a spin-valve film. Among these materials, Ag and Au are more preferable, and Ag is most effective. In the case of using Ag or Au, the (111) plane is more likely to be smooth and it is easier to obtain a surface which can be regarded as smooth on the order of several tenths of a nm. Thus, the (111) plane is preferably substantially parallel to the surface of a substrate.
More preferably, a non-magnetic layer made of Cu or the like is inserted between the metal reflective layer and (a magnetic layer of) the spin-valve film. The non magnetic layer not only functions as a buffer layer for smoothing the surface of the metal reflective layer, but also increases the spin-dependent scattering at the interface between the non-magnetic layer and the magnetic layer.
Moreover, it is also preferable to provide a Co layer between the magnetic layer and the metal reflective layer. The layer is provided for increasing the MR ratio by enhancing the spin-dependent scattering in the interface between the magnetic layer and the non-magnetic layer (i.e., the metal reflective layer).
More preferably, the entire spin-valve film is epitaxially grown over a single crystalline substrate.
Furthermore, the magnetoresistive device of the present invention is preferably configured such that the magnetization-easy axis of a soft magnetic layer in a magnetoresistive device portion is vertical to the direction of a signal magnetic field to be detected.
The magnetoresistive head of the present invention is characterized by further including a lead portion, in addition to the magnetoresistive device.
Thus, the invention described herein makes possible the advantages of (1) providing a magneto resistive device having a high MR ratio and a magnetoresistive head using the same, (2) providing a spin-valve type magnetoresistive device in which electrons have a long mean free path and a magnetoresistive head using the same, and (3) providing a spin-valve type magnetoresistive device, In which the possibility of the spin-dependent scattering generation of electrons is high at an interface between a magnetic layer and a non-magnetic layer and a magnetoresistive head using the same.